Regarding exothermic electronic parts such as power devices, double-side heat dissipation transistors, thyristors, CPU and the like, efficient heat dissipation during their use is important. Generally, conventional measures for such heat dissipation were to (1) improve thermal conductivity of the insulating layer of a printed-wiring board onto which the exothermic electronic parts are to be mounted, and (2) mount the exothermic electronic parts or the printed-wiring board having the exothermic electronic parts mounted thereon onto a heat sink via a thermal interface materials having electric insulation or a ceramics insulating plate. As the insulating layer of the printed-wiring board and the thermal interface materials, heat dissipating member obtained by curing silicone resin and epoxy resin added with ceramics powder is used.
In recent years, higher speed and higher integration of the circuit in the exothermic electronic parts and higher density of the exothermic electronic parts being mounted onto the printed-wiring board have lead to higher heat generation density and more precise structure in the electronic devices. Accordingly, heat dissipating member having even higher thermal conductivity has been required.
From the afore-mentioned circumstances, hexagonal boron nitride having superior properties as the electric insulating materials such as (1) high thermal conductivity, (2) high electric insulating property and the like have been receiving attention. However, while the thermal conductivity of boron nitride is 100 to 400 W/(m·K) in the in-plane direction (direction in the a-axis), the thermal conductivity in the thickness direction (direction in the c-axis) is 2 W/(m·K). Accordingly, anisotropy of the thermal conductivity derived from the crystalline structure and the flake shape is large. Accordingly, for example, when the thermal interface materials is manufactured, the in-plane direction (direction in the a-axis) of the boron nitride particle and the thickness direction of the thermal interface materials can come vertical with each other, thereby resulting in cases where the high thermal conductivity of the boron nitride in the in-plane direction (direction in the a-axis) could not be utilized sufficiently.
(First Viewpoint)
High thermal conductivity of the boron nitride particles in the in-plane direction (direction in the a-axis) can be achieved by allowing the in-plane direction (direction in the a axis) of the boron nitride particles and the thickness direction of the thermal interface materials to come in parallel with each other, however, there is a problem in that the material is weak in the tensile stress in the thickness direction.
In Patent Literature 1, a resin composite material containing high rigidity particles such as ceramics and metals by 40 to 90% by volume fraction, the high rigidity particles being in contact with each other three-dimensionally, and a manufacturing method thereof are disclosed. Here, it is also disclosed that such resin composite material can be suitably used in mechanical parts such as a slide member exemplified as a wire saw roller and a gear wheel.
In addition, Patent Literature 2 discloses a sintered ceramics member having at least forsterite and boron nitride as its main component and having boron nitride aligned in one direction, a probe holder formed by using the ceramics member, and a manufacturing method of the ceramics member. Here, it is also disclosed that such ceramics member can be suitably used as a material for a probe holder, into which a probe is inserted. Here, the probe is for a micro contactor used in inspection of semiconductors and liquid crystals, and electrically connects a circuit structure to be inspected and a circuit structure which output signals for inspection.
Patent Literature 3 discloses a method in which a filler having a large anisotropy in shape or in thermal conductivity is mixed and dispersed in a thermosetting resin material, followed by curing of the thermosetting resin. Subsequently, the cured thermosetting resin is crushed, mixed with a thermoplastic resin to obtain a resin composition for a mold, and then the resin composition is heated to soften and mold the resin composition into a desired shape.
Patent Literatures 4 and 5 disclose of a method for manufacturing a substrate for a printed circuit comprising the step of impregnating a thermal setting resin (II) into an inorganic continuous porous body selected from the group consisting of an aluminum nitride-boron nitride composite (AIN—BN), an alumina-boron nitride composite (Al2O3—BN), a zirconium oxide-boron nitride composite (ZrO2—BN), silicon nitride-boron nitride composite (Si3N4—BN), hexagonal boron nitride (h-BN), β-wollastonite (β-CaSiO3), mica, and volcanic soil; followed by curing to obtain a cured plate body. In addition, it is also disclosed that such substrate for a printed circuit can be suitably used as a substrate for high frequency usage and for directly mounting a semiconductor chip.
Patent Literature 6 discloses a porous body of B—C—N system having a graphite three-dimensional skeletal structure synthesized from a porous polyimide sheet as a starting material, and a heat dissipating material obtained by impregnating a resin in the porous portion to obtain a composite material. Such heat dissipating material has a smaller heat resistance compared with those obtained by impregnating a resin in an ordinary carbon porous body, and an insulating composite material can be obtained by the conversion of the porous body into h-BN. Accordingly, the heat dissipating material is a promising material as a cooling material for electronic parts which require low heat resistance and electric insulating property.
(Second Viewpoint)
In addition, not only the heat dissipation in one direction of the thickness direction or the plane direction as conventionally required, but also high heat dissipation in both of the thickness direction and the plane direction is required.
Patent Literature 7 discloses a substrate for an electronic circuit comprising a ceramics composite obtained by filling a resin in open pores of a porous ceramics sintered body, the porous ceramics sintered body having a three-dimensional network crystalline structure. Here, the porous ceramics sintered body is structured with a ceramics material of which crystal grains have an average crystalline grain diameter of 10 μm or less. However, it is difficult to align the flake-like boron nitride particles randomly with the method of Patent Literature 7. Accordingly, anisotropy of thermal conductivity could not be decreased.
When the method of Patent Literature 2 was used, the orientation degree of the flake-like boron nitride was large, showing I.O.P (The Index of Orientation Performance) of 0.07 or lower. Accordingly, the anisotropy of thermal conductivity could not be decreased.
In the method of Patent Literature 3, thermal conductivity was low as showing a maximum value of 5.8 W/(m·K). In addition, the thermosetting resin needs to be crushed after being obtained, and then the thermosetting resin is mixed and softened again. Accordingly, it was problematic in the viewpoint of reliability due to the possibility of contamination and uniformity of the softening condition of the resin.
Patent Literature 8 discloses of increasing the temperature of the mold when the resin molding is performed, thereby making random the direction of heat dissipation of the inorganic filler. However, in the method of Patent Literature 8, alignment of the inorganic filler can be controlled only insufficiently, and thus the decrease in the anisotropy of the thermal conductivity was insufficient.
Patent Literature 9 discloses of a method in which the manufacturing conditions of the boron nitride is adjusted to allow the flake-like boron nitride to aggregate, thereby obtaining a pinecone-like boron nitride powder. However, in the method of Patent Literature 9, the pinecone-like aggregated particles of the boron nitride would partly align during the coating process and the heat-pressing process performed in the processes to manufacture a thermoconductive sheet, and thus the decrease in the anisotropy of the thermal conductivity was insufficient.
Patent Literature 10 discloses of impregnating a ceramics powder slurry in a boron nitride sintered body and a composite sintered body, thereby achieving dust free properties. However, since the boron nitride sintered body and the composite sintered body of Patent Literature 10 are manufactured generally by powder molding or hot press, alignment of boron nitride cannot be avoided, and thus there was anisotropy in the thermal conductivity.
Patent Literature 11 discloses a thermoconductive sheet comprising plate-like boron nitride particles and an organic polymer compound having a glass transition temperature (Tg) of 50° C. or lower; the plate-like boron nitride particles being aligned in the direction of longitudinal axis thereof with respect to the thickness direction of the sheet. Here, regarding the thermoconductive sheet obtained by the method of Patent Literature 11, although the thermal conductivity of the thermoconductive sheet in the thickness direction is as high as 26.9 W/(mK) at maximum, the plate-like boron nitride particles are aligned, and thus there was anisotropy in the thermal conductivity.
In the conventional technique, the heat dissipating member is manufactured via a mixing process to mix the ceramics powder such as boron nitride and the resin, an extrusion molding process, a coating process, a heating process and the like. Therefore, it is difficult to avoid the alignment of the boron nitride crystals. Accordingly, the decrease in the thermal conductivity was limited. The issue of alignment can be suppressed when spherical particles of aluminum oxide powders and silicon oxide powders are used, however, thermal conductivity of these ceramics powders is approximately 20 to 30 W/(m·K) and is lower than that of boron nitride. In addition, since the particles are hard, there was a problem in that the apparatus and the mold would be worn down. Further, in the heat dissipating member manufactured by conventional technique, thermoconductive fillers such as boron nitride were added in the form of powders, and thus it was necessary to increase the filling amount of the thermoconductive fillers up to approximately 60% by volume. However, such technique would raise the cost, and the thermal conductivity of the heat dissipating member was 6 W/(m·K) or lower, resulting in difficulty in meeting the recent demands for higher thermal conductivity.
In the electronic parts using the heat dissipating member, when the heat dissipating member is a conventional one having large anisotropy in thermal conductivity, the arrangement of the cooling unit and the heat transport unit would be limited, thereby resulting in cases where further miniaturization of the electronic device becomes difficult. Accordingly, development of a dissipating member having superior thermal conductivity and small anisotropy in the thermal conductivity is strongly desired.
Regarding these problems, a heat dissipating member having superior thermal conductivity and small anisotropy in the thermal conductivity can be manufactured by using a resin-impregnated boron nitride sintered body containing a resin. Here, in the boron nitride sintered body, flake-like boron nitride particles having a specific calcium content ratio, specific graphitization index of the boron nitride, and suitably controlled average grain size are allowed to have a three-dimensional bonding with small orientation degree in the boron nitride crystals, thereby enhancing the accessibility among the boron nitride particles. However, there has been no technical suggestion provided in these points of views.